Harmful Benthic Algal Bloom (HBAB)

Ostreopsis grows epiphytically on macroalgae, although they can sometimes be found growing on sediments. In Italy, harmful events related to Ostreopsis blooms were first recorded on the coast of Tuscany in 1998, and have since been reported in many other coastal Italian areas and normally during summer. In some cases there has been a conspicuous, thick, brownish mucilage layer covering all surfaces.

The cells of the microalgae in these cases are found embedded within this mucilage. Occasionally, mucilage flocs containing a high number of cells may detach and enter the water column causing more widespread problems.

Sites where Ostreopsis has been recorded are shown in the following map (colored circles). The name of each site appears when the mouse pointer stays a few seconds over the corresponding symbol.
 

The ecological impacts caused by Ostreopsis blooms may be dramatic. Blooms may affect water quality, cause hypoxia and/or anoxia or, more seriously, mortality in benthic invertebrates such as molluscs, coelenterates and echinoderms. In addition, there can be quite serious effects on human health, effects that have raised public concern in the last few years in several Italian localities. For example, more than 200 people were hospitalised due to respiratory problems and conjunctivitis in 2005 in Liguria, whereas in Apulia in 2004 there were several reported cases of dermatitis and temperature alterations.

These symptoms were associated with the presence of toxins released into seawater and into the atmosphere by Ostreopsis ovata (after analyses of natural samples of macrophytes covered with Ostreopsis and confirmed by molecular based assays). During recent blooms, palitoxin-like compounds have been detected in plankton and water samples. Palytoxin is among the most toxic natural substances known. In some cases, it has been associated with human poisonings following fish or crustacean consumption.

Poisonings caused by Ostreopsis spp. along the Italian coasts were quite similar to those of Ciguatera Fish Poisoning (CFP) events caused by benthic microalgae in the tropics. In the case of ciguatera, microalgae of the dinoflagellate genus Gambierdiscus colonise macroalgae that are fed upon by herbivorous fish. These fish accumulate ciguatoxins and, when eaten, provoke a chronic neurologic syndrome. Fortunately, species of Gambierdiscus have not been recorded in the Mediterranean so far. Tropical benthic ecosystems clearly differ from those of the Mediterranean Sea; however, the general conditions in the latter are closer to subtropical than to temperate conditions that are found at similar latitudes. This results from the presence of the Gibraltar ridge, which prevents the colder waters of the Atlantic (< 13 °C) to enter into the Mediterranean, and may be enhanced by ongoing climatic change. What is differing here with regard to the situation involving Gambierdiscus, is that the transfer pathway of toxins produced by Ostreopsis spp through the trophic web is unknown. In addition, no records of toxic aerosols have ever been associated with ciguatera toxins. 

Benthic dinoflagellate associations in Italian Seas include other potentially toxic species that colonise macroalgae: Prorocentrum lima, which produces okadaic acid, a toxin responsible for Diarrhetic Shellfish Poisoning (DSP) and Coolia monotis, that produces toxins yet to be identified and whose effects on humans are unknown.